The present invention relates to a process for producing a hybrid flexible hose made of synthetic resin such as nylon and rubber, and more particularly, to a process for producing a long sized flexible hose without employing a solid mandrel of polyamides or the like.
Though the hose made through the process of the present invention can be applied in various field, the hose is especially useful for service of refrigerant, e.g. Freon, in an air conditioner of an automobile or for service of liquid fuel, e.g. gasoline or light oil.
As to a process for producing the hybrid hose made of synthetic resin and rubber, there has been hitherto known a process employing a mandrel mentioned hereinafter.
That is, the conventional process comprises a first step of preparing an unvulcanized hose by providing a solid mandrel as a core; forming an inner tube or core tube, for example, by extruding synthetic resin, e.g. polyamide resin, on the mandrel; applying unvulcanized rubber material to be a middle rubber layer on the core tube; braiding a reinforcement layer to cover the middle rubber layer; and applying an outer rubber layer thereon. Then secondly vulcanizing of the intermediate product is performed, and at last, the mandrel is extracted from the vulcanized hose.
In the above-mentioned conventional process, a metal mandrel, for example made of steel or stainless-steel, a resin mandrel, a rubber mandrel, and the like is employed as a solid mandrel. In addition, the layer of reinforcement strand is braided in neutral angle or rest angle (i.e. 54.degree.44') at first in order to progress the pressure resistance of the hose.
However, the above-mentioned conventional process has the following drawbacks.
When a metal mandrel is employed as a solid mandrel, a molten resin is firstly extruded by means of a cross-head die and applied on the mandrel, or a previously prepared resin tube is mounted on the mandrel. Therefore, length of the hose is restricted by the factory space, since at least double length is required as an equipment space, and about 20 m is the maximum length for a producible hose. Further, when a short hose is produced, such conventional process causes disadvantages that production cost is high, and efficiency of work is low, since the mandrel is heavy.
When a resin mandrel or rubber mandrel is used, the mandrel is not easily extracted from a vulcanized hose, especially when the hose is long. In order to smoothly extract the mandrel, mold release agent or such lubricant is previously applied on the mandrel. However, if the agent is excessively applied, water in the agent evaporates and expands during vulcanization, and therefore, pin-holes or thin portions are partially caused in the resin core tube which is the innermost layer of the hose. Further, when the resin core tube has a wall thickness less than about 0.4 mm, the core tube tightly fits on the mandrel. Therefore, the inner tube is deformed or torn when the mandrel is extracted from a completed hose.
Since the conventional process employing a mandrel has the above-mentioned drawbacks, long-sized hoses (e.g. having a length not less than about 20 m) cannot be produced with low cost and quality of the products is not stable.
On the other hand, an internal air pressure might be employed instead of a solid mandrel as is employed in a field of low pressure hose production. However, as to high pressure hose having a reinforcement strand braided at rest angle, it is required to keep remarkably higher internal pressure than the steam pressure in an autoclave during vulcanization. That is, when the internal pressure is low, the inside surface of the core tube deforms to a square shape and becomes uneven. On the contrary, when the internal pressure is high, it is very difficult to seal the pressure in the core tube due to plastic flow of the core tube and rubber layers under the high temperature during vulcanization, and therefore, the inside diameter of the hose cannot be finished to a predetermined size due to leak of the internal pressure.
An object of the present invention is to provide a new producing process by which a long-sized hybrid hose can be obtained with low cost and an inside diameter can be correctly finished to a predetermined size.